It is well known that highly condensed aromatic hydrocarbons and dioxines are formed in combustion processes, particularly in the incineration of garbage, and are discharged from the combustion system together with the filter dust and the exhaust gas. The general term "highly condensed aromatic hydrocarbons" describes multinuclear aromatic compounds, which may also contain one or more OH groups. The general term "dioxines" is used to describe polychlorinated di-benzo-p-di-oxines and polychlorinated dibenzofuranes. The dioxines include a total of 210 isomers, some of which are extremely toxic. The highly condensed aromatic hydrocarbons are also deleterious to health because they are regarded as cancerogenic substances. Owing to the toxicity of highly condensed aromatic hydrocarbons and dioxines, said compounds must be removed from the filter dust and from the exhaust gas or the formation of said compounds in the combustion system must be avoided.
It is known that highly condensed hydrocarbons, which may contain also OH groups, are formed in combustion systems at 300.degree. C. to 400.degree. C.--during the cooling of the exhaust gases--by a recombination of organic radicals in the presence of oxygen. It is also known that dioxines are formed below 300.degree. C. in combustion systems because chlorine is formed from the existing chlorine compounds in the presence of oxygen at higher combustion temperatures and said chlorine will cause the dioxines to be formed in the presence of organic compounds or carbon as the exhaust gases are cooled. That reaction is additionally catalyzed by the heavy metal compounds (oxides) which are contained in the flue dust. For this reason it has already been proposed to separate the flue ash from the exhaust gas stream from combustion systems at temperatures above 300.degree. C. because it is known that highly condensed aromatic hydrocarbons and dioxines are thermally unstable above 600.degree. C.
In accordance with Published German Application 3,644,381, the emission of pollutants from combustion systems is minimized by a process in which the flue gases are dedusted at temperatures of about or above 300.degree. C. immediately upon discharge from the furnace before the heat exchanger or in the first portion of the heat exchanger. The dedusting is affected in a separating nozzle (centrifugal deduster) that is made of ceramic material or in a granular bed filter. The process proposed in Published German Application 3,644,381 is intended to prevent a catalytic formation of dioxines owing to the presence of particles of flue dust in the flue gas as it is cooled.
It has been found that a granular bed filter and a centrifugal deduster cannot dedust the exhaust gas from the combustion to such a degree that a formation of highly condensed aromatic hydrocarbons and dioxines will always be precluded. Investigations have shown that the pure gas from combustion systems in which a dedusting is effected by a granular bed filter and/or a centrifugal deduster will at least temporarily contain dust in excess of 5 mg/sm.sup.3 (sm.sup.3 =standard cubic meter) and that even such low dust contents will be sufficient for a catalytic formation of highly aromatic hydrocarbons and particularly dioxines.
For this reason it is an object of the invention to provide a process by which a formation of highly condensed aromatic hydrocarbons and dioxines in combustion systems will be avoided and which is preferably safe and will reliably prevent a formation of the above-mentioned substances during the cooling of the exhaust gases so that said substances can no longer be detected in the filter dusts and the exhaust gases. It is another object of the invention to provide an apparatus for carrying out the process.